The present invention relates generally to inverter control systems and, more particularly, to a system and method for controlling an inverter to operate within a safe operating area while maximizing a power output from an alternative energy source such as, for example, a photovoltaic (PV) array, to a power grid.
Most homes and/or facilities with electrical systems receive alternating current (AC) electrical power from a utility power grid. Many facilities connected to utility power grids will substitute power from the power grids with power from their own alternative energy sources. The alternative energy sources may include solar, wind, geothermal, and/or hydroelectric energy sources, as non-limiting examples. In some cases, the alternative energy sources will generate power in excess of what the facilities need to operate the facilities' electrical systems. In those cases, the excess generated electrical power from the alternative energy sources can be supplied back into power grids in exchange for compensation.
Since power grids provide AC electrical power, only AC power can be supplied into power grids. Therefore, in the case of alternative energy sources that produce direct current (DC) power, an inverter must be used to invert the energy from the DC alternative energy sources from DC power to AC power. An inverter is operated by a controller that selectively controls switches of the inverter to invert the DC power into AC power. During operation, the inverter controller receives a reactive power command from a utility and a power grid voltage from the power grid to which the inverter supplies power. The controller then regulates switching within the inverter to supply that reactive power and voltage to the power grid.
These inverters are controlled to limit the input DC-side current and the output AC-side current so as not to damage the internal hardware of the inverter. The operation of an inverter is also restricted by the DC voltage of the alternative energy source. Typically, the controller for an alternative energy source inverter is designed to switch off the inverter if the DC voltage of the alternative energy source drops below a preset threshold DC voltage. Therefore, when the DC voltage of the alternative energy source drops below the threshold, no active or reactive power is supplied to the power grid. Further, the reactive power specified for an inverter is limited to a small percentage of the apparent power rating of the inverter (for example, an inverter may be restricted to providing a power factor of +/−0.91). All of the above limitations—the inverter DC and AC current limits, the reactive power command and the power grid voltage, the alternative energy source DC voltage, and the inverter power factor limits—impose restrictions on extracting the maximum amount of active power out of alternative energy sources.
The above limitations pose additional restrictions when the inverter is used for higher apparent power applications. Because an inverter has limited AC current capabilities, the AC voltage of the inverter must be increased to increase the apparent power rating of the inverter. However, the DC voltage supplied to the inverter from the alternative energy source remains the same. This increase in the AC voltage of the inverter makes the problem of low DC voltage more severe because the inverter must be disconnected from the alternative energy source at a higher DC voltage, further limiting the inverter from extracting active power from the alternative energy source and supplying the requested reactive power to the power grid.
In any case, a preset (static) threshold DC voltage limits an inverter from providing any power to a power grid when the DC voltage generated by the alternative energy source is below that threshold DC voltage. Therefore, even if the alternative energy source has a DC voltage output that could be used to provide power to the power grid, the inverter will be switched off if the DC voltage is below the threshold.
It would therefore be desirable to provide a system and method for controlling an inverter to operate within the hardware limitations of the inverter and the limitations of a power grid while maximizing the active power provided to the power grid from an alternative energy source under low-DC voltage conditions.